The proposed research is directed toward solution of the problem of how genes dictate development in higher animals. Drosophila melanogaster is the preferred organism for attempting this solution because its genome is among the smallest and best defined, because more genes specifying development have been isolated, and because cloned DNAs can be easily inserted into its germ line. This grant focuses on the genetic regulatory functions of proteins encoded by the homeotic Ubx gene of the bithorax complex (BX-C) and on the regulation of this gene by the products of other genes. This gene provides the major determinants specifying the developmental pathoways for the identities of the metameres comprising the posterior half of the torax and the most anterior part of the abdomen (parasegments (ps) 5 and 6). The Ubx gene consists of a 78 kb transcription unit whose transcripts are differentially spliced to yield at least five mRNAs encoding a family of Ubx proteins characterized by constant N- and C-terminal regions and a variable central region lying just upstream from a homeo domain encoded by the conserved homeo box. One goal of the project is to define the mechanisms controlling his differential splicing. Another is to define the mechanisms by which each of these proteins binds to and regulates the expression of their target genes, two of which are known: the Antp homeotic gene and the Ubx gene itself. A connected goal is to define the other target genes of these regulatory proteins. Cis-acting elements controlling the transcription of Ubx are spread over a 40 kg region (bxd region) lying upstream of the gene. This control region and the Ubx gene comprise one third of the BX-C, whose remainder is occupied by two other complex genes (abdA and AbdB), each containing a homeo box. Another goal of this project is to define the mechanisms by which the bxd control region, and another cis- regulatory region in an intron of Ubx, regulate Ubx transcription, and particularly, how proteins encoded by abdA and abdB, as well as by certain segmentation genes (e.g., ftz) and by ubx itself, modulate this cis-regulation. There is thus an increasing emphasis on regulating proteins in this project. The development of methods for the synthesis and isolation of these proteins from Drosophila host-vector expression systems, and of novel techniques for determining their regulatory functions, is a further aim of this project.